Climate Variability and Change in Mobile, Alabama

7. Storm Events

Severe storms, such as hurricanes, can have temporary, but unpredictable and highly damaging effects. These effects include temporary surges in sea level (lasting several days) that can inundate coastal areas, precipitation-induced flooding, strong wind, and waves, all potentially damaging to infrastructure. Hurricanes have had severe impacts on Mobile in the past. For example, in 1979, Hurricane Frederic caused approximately $1.7 billion (1979 USD) in damage and wiped out sections of the causeway linking Dauphin Island to the mainland.1

According to a scientific assessment from the U.S. Climate Change Science Program, "the power and frequency of Atlantic hurricanes have increased substantially in recent decades, though North American mainland land-falling hurricanes do not appear to have increased over the past century. …There is evidence suggesting a human contribution to recent changes in hurricane activity as well as in storms outside the tropics. …Hurricane wind speeds, rainfall intensity, and storm surge levels are likely to increase [in the future]".2 In other words, there are likely to be more large hurricanes in the future. However, due to the relatively infrequent nature of hurricanes it is difficult to identify when or whether such an increase would be detected in Mobile.

Other severe storms, such as mid-latitude storms and thunderstorms, can also produce significant rain and cause severe damage. The damage associated with these storms has increased over time, in part, due to the growth in population and infrastructure.3 According to a scientific assessment from the National Academy of Science, "Changes in major storm events are of interest both because a significant fraction of total U.S. precipitation is associated with storm events and because storms often bring wind, storm surges, tornadoes, and other threats. … Extratropical storms, including snowstorms, have moved northward in both the North Pacific and North Atlantic, but the body of work analyzing current and projected future changes in the frequency and intensity of these storms is somewhat inconclusive. Historical data for thunderstorms and tornadoes are insufficient to determine if changes have occurred."4 Projecting changes in mid-latitude storms and thunderstorms is an area of active research; the main findings relevant to Mobile, Alabama are presented here.

In this section, the methodology for evaluating observed storm events in the Mobile region and storm event projections is provided followed by a description of key findings.

Section 7.1 describes the characterization of observed storm events, including discussion of five representative storm events as case studies. The case studies identify and characterize the extreme events that impact the study area. This provides an understanding of the current weather hazards that affect transportation planning and design. The case studies also identify key environmental phenomena that were crucial for developing the extreme storm. This is useful for then understanding how these extreme storms may change in the future. Information on reported damage from each storm was also recorded in the case studies; this information will help inform the vulnerability assessment in later stages of the project.

Section 7.2 describes the analysis of future storm events. This analysis included a literature review of studies projecting how storm-producing atmospheric phenomena may change and a scenario- and model-based analysis of hurricane storm surge.

Section 7.3 discusses the implications of these findings on transportation.

Additional detail about the storm event analyses is available in the appendices.

7.1. Observed Storm Events

This section discusses the types of storms that Mobile experiences, and investigates five representative storm events that have previously occurred in Mobile. These case study storms provide context for understanding the impacts that past storms have had on Mobile's transportation assets and services. This section also highlights the meteorological conditions, such as the placement of the jet stream, that were important in the development of each storm event. Section 7.2 discusses how these key meteorological conditions may change in the future, providing context as to how these case study storms could change in the future.

7.1.1. Methodology

Mobile, Alabama experiences a large variety of storm events. To help characterize historical storm events in the Mobile region, the National Weather Service (NWS) office in Mobile provided a list of recent local storm events (this study focuses on those events occurring from 1995 onward). The list consisted of 18 mid-latitude storms and thunderstorms (i.e., storms other than tropical storms or hurricanes) and 16 tropical storm and hurricane events. The list was supplemented by a targeted literature search to determine if additional research was available that could enhance the analysis.

The NWS list and literature search results were used to characterize the types of storms occurring in Mobile, as well as the meteorological conditions leading to and experienced during them. The list was then used to develop a representative set of case studies to investigate local storm events.

To select storms for case studies, the storm events were first organized by storm type and level of impact to ensure that the analysis covered the variety of storm events that affect Mobile. Each storm was then evaluated based on:

(1) Whether the storm was a good representation of the types of storms that hit Mobile,

(2) Whether sufficient information was readily available to develop a case study, and

(3) Whether the storm type was likely to occur under future projections.

Five storm events were selected for case studies. These events include:

A literature survey was then conducted for each storm event to provide additional information on storm analysis, damage information, and general meteorological conditions contributing to storm development and/or intensification.

The mid-latitude storms and thunderstorm case studies include:

A brief discussion of storm development identifying key meteorological conditions;

7.1.2. Key Findings

Key Findings for Historical Storms

Mobile experiences frequent severe thunderstorms in the spring and fall, often accompanied by tornadoes. Key ingredients for these strong convective storms are a strong jet stream and warm, moist surface air.

Mobile is also frequently affected by tropical storms and hurricanes, including 12 storms since 2000. Mobile receives a direct hit about once every 16 years.

Storm events in Mobile can cause flooding, downed power lines, and other infrastructure damage.

General Characterization

Prior to investigating specific case studies, storm event types in Mobile were characterized more generally, including the meteorological conditions leading to, and experienced during, each type of storm. Storm events in Mobile have been characterized into two types corresponding to the sections below: (1) mid-latitude storms and thunderstorms, and (2) tropical storms and hurricanes.

Mid-Latitude Storms and Thunderstorms

Mid-latitude storms and thunderstorms are a common occurrence in Mobile during the summer months. The southerly direction of the prevailing wind transports warm moist air from the Gulf of Mexico into southern Alabama. This warm moist air rises into the atmosphere, condenses, and creates air-mass thunderstorms.

In the summer, it is unusual for these air-mass thunderstorms to develop into severe thunderstorms. These thunderstorms occur locally and are missing a few key ingredients: wind shear; a strong trigger for significant uplift of warm, moist air; and divergence aloft. These summer air-mass thunderstorms are not associated with tornadoes.8

In the spring and fall months, however, these key ingredients are available: a prevailing southerly wind providing a source of warm moist air, the periodic presence of cold fronts rolling into Alabama providing the necessary uplift mechanism, and, in some cases, the jet stream loops far into the southern United States providing significant divergence aloft. Severe thunderstorms tend to develop as much as 100 or more miles (160 or more kilometers) ahead of the cold front.9

Severe spring thunderstorms are most common in March, April, and May between noon and 7:00PM, producing tornadoes, hail, and strong winds.10 A second season of thunderstorms occurs in the fall, from late October through December with severe storms producing tornadoes. Tornadoes occur most often in November (see Figure 56). Hail may be present and, depending on the strength of the wind, may be particularly damaging. Table 21 presents a list of severe thunderstorms recently affecting Mobile.

Figure 56: Tornadoes by Month and Hour for the State of Alabama from 1950 to 2005

Source: RMS, 2009

The Role of the Jet Stream

The polar jet stream plays an important role in generating extreme storm events in Mobile, Alabama. The polar jet stream is a fast moving stream of air about 10,000 feet above the surface of the Earth, traveling from west to east across the United States. The jet stream occurs between cold northern Arctic air to the north and warm moist southern air to the south. Because storms draw much of their energy from temperature differences, this boundary between cold and warm air masses is a highly favorable location for storms. In addition, the jet stream acts as a source of vertical wind shear also highly favorable to storm development.

The polar jet stream can travel south towards Alabama from fall through winter and into early spring. The jet stream brings a cold front associated with a mid-latitude cyclone (or low pressure system) into Mobile about once per week. This cold front typically dominates the weather for several days and is replaced by cold sunny days until the next cold front comes in.

When the jet stream travels south, air masses steered by the moving mid-latitude cyclone enter Alabama. A warm, dry air mass enters first. Being denser than the prevailing warm, moist air from the Gulf of Mexico, the warm, dry air mass pushes the warm moist air aloft, creating instability. The cold air mass enters Alabama next. Because the cold air mass tends to travel faster than the warm air mass, the cold front pushes less-dense warm air up as it advances, causing a rapid uplift (see Figure below). This can result in a squall line of severe thunderstorms that can spawn tornadoes. A squall line can last 12 hours or more.

Within this squall line, a supercell can be generated. A supercell is a long-lasting thunderstorm that brings flash flooding, damaging hail, wind, and families of tornadoes. Supercells tend to develop in late-winter and spring. A 150-mile (240-kilometer) wide tornado line from the southwest corner of Alabama to the northeast corner is the most active in the region for tornadoes.

An Example of the Boundary between Cold and Warm Air Masses and the Resultant Convection

Source: RMS, 2009

An example of a boundary between cold and warm air masses and the resultant convection: (1) First, cold air moves into warmer air and cuts beneath it; (2) then, warm air is forced to rise and overtops the encroaching cold air; and (3) finally, rising air creates clouds and stormy conditions.

Due to Alabama's temperate climate, snow is rare in Mobile. Snow generally occurs due to northern Arctic air entering Alabama and hitting the warm, moist Gulf air.11 Thunder and lightning during a snow event generally indicates that a strong low pressure system is pulling warm air from the Gulf of Mexico over the cold air at the surface.12

Recent Seasonal, Non-Tropical Storm Events Affecting Mobile, Alabama

Heavy rainfall event, April 4-5, 2008

Damaging winds, February 12, 2008

Rare snow storm, January 19, 2008

Gulf Coast hard freeze, December 5, 2006

New Year Snow , January 1, 2002

Severe thunderstorms can also occur during the winter, as evidenced by the occurrence of tornadoes.

Freezing rain can occur during the winter months when surface temperatures are low and raindrops freeze on impact.13

Periods of extreme heat, extreme cold, or even drought14 can occur in Alabama when a stationary front stays in the area. The stationary front can shift the prevailing wind direction so that moist air from the Gulf does not enter Alabama. This occurs most frequently in the winter.15 Conversely, during the summer months, drought can occur in Mobile when a high pressure system remains in the area for weeks and blocks the warm moist air from the Gulf.16

Finally, advection fog can impact Mobile during the winter months. Dense advection fog occurs as warm, moist air from the Gulf of Mexico travels over cold land.17

Tropical Storms and Hurricanes

Recent Tropical Storms and Hurricanes Affecting Mobile, Alabama

Tropical Storm Lee, September 4, 2011

Tropical Storm Ida, November 10, 2009

Hurricane Ike, September 13, 2008

Hurricane Gustav, September 1, 2008

Tropical Storm Fay, August 23, 2008

Hurricane Katrina, August 2005

Hurricane Dennis, July 2005

Hurricane Cindy, July 7, 2005

Tropical Storm Arlene, June 11, 2005

Hurricane Ivan, September 2004

Tropical Storm Barry, August 2001

Tropical Storm Helene, September 2000

Tropical Storm Hermine, September 1998

Hurricane Georges, September 1998

Hurricane Danny, July 1997

Hurricane Opal, October 1995

Hurricane Erin, August 1995

Source: NOAA NWS, Mobile Office

In the list provided by the Mobile NWS office (see textbox), all of the summer storm events classified as extreme are tropical storms and hurricanes.18 Though the Atlantic hurricane season runs from June 1 through November 30, hurricanes primarily affect Alabama in May, June, mid-September, October, and November.19 Warm sea surface temperatures (SST) from the Gulf Stream crossing a section of the Gulf of Mexico increase the likelihood that tropical cyclones will intensify and occur. This occurred in 2005 with Hurricanes Katrina, Rita, Wilma, and others.20 Once developed, about 25% of Gulf-Atlantic tropical cyclones hit the mainland. Multiple strikes can occur within a given season.21

Over the twentieth century, Alabama experienced 17 direct hits from hurricanes, including Frederic (category 4) at Dauphin Island in 1979, Ivan (category 3) in 2004.22 Figure 57 illustrates the hurricanes and tropical storm strikes experienced in Mobile since 1980. Alabama experiences a storm23 that originated in the tropics approximately every 1.5 years. Hurricanes impact Alabama about every 7.5 years.

Figure 57: Storm Tracks of Hurricanes and Tropical Storms that Have Impacted Mobile, Alabama over the Past 15 Years

Figure 58 displays the timing of hurricane strikes in relationship to the population of Mobile County, Alabama. The hurricanes are marked by category number with green labels for category 1 and 2 hurricanes, and red labels for stronger hurricanes. The population of the county represented by the bars has increased substantially since 1900, increasing more than 50% from 1940 to 1960. A number of direct and indirect hurricane strikes occurred between two time periods: 1900 to 1930 and 1980 to 2000. The figure suggests more hurricane strikes have occurred in the past few decades (1980-2000) than during any previous twenty-year period in the twentieth century.

Storm surge associated with tropical storms and hurricanes can cause significant coastal damage. A recent study developed storm surge return periods for the U.S. Gulf Coast based on an analysis of available data and other information dating back to 1880.24 A surge database, SURGEDAT, provides the results of this analysis. Table 22 summarizes the findings in SURGEDAT for Alabama and the western Florida Panhandle.

Case Studies

As discussed in the methodology section above, five different storm events were analyzed to identify the key characteristics of the storms and associated damages in Mobile. These storms represent a sampling of the different types of storms that Mobile experiences, including a thunderstorm and tornado event, a hailstorm, a heavy rain event, Hurricane Georges, and Hurricane Katrina. The case studies are summarized in detail in Appendix D.6. Abbreviated summaries are presented in this section.

Storm Development

Severe thunderstorms strong enough to produce six tornadoes struck the Mobile region on November 15, 2006.26 These thunderstorms developed due to a strong southerly jet stream aloft that steered a low pressure system into Alabama. Key meteorological conditions for this storm's development were: (1) a strong jet stream aloft, (2) a surface cold front associated with a low pressure system, and (3) warm, moist surface air. As detailed earlier, this is a typical example of a severe storm event in Mobile, Alabama.

Storm Highlights

Severe thunderstorms produced six tornados

Wind gusted at over 50 mph

Five flash warnings were issued

Rain totaled up to 8 inches across the region

Most rain fell in a 3-6 hour window

Storm Damage

Strong winds and tornadoes caused the majority of storm damage. Debris, fallen trees, and downed power lines blocked roadways. Flooding also impacted transportation infrastructure. The NWS estimates the storm's six tornadoes caused $0.5 million to $1 million of damage.27

Extreme Event Comparison

Observed 24-Hour Precipitation: greater than a 2 year event

Observed Peak Discharge: greater than a 2 year event

Observed Storm Surge: less than a 10 year event

Case Study 2: Severe Hailstorm, March 5, 1998

Storm Development

Thirteen severe thunderstorms developed in the Mobile region on March 5, 1998. The storms brought hail ranging from the size of a dime to the size of a baseball.28 Key meteorological conditions leading to the storm's development include: (1) a strong west-to-east jet stream aloft, (2) cold, dry air in the middle layer of the atmosphere, (3) vertical wind shear, (4) strong potential for convective thunderstorms,29 and (5) a high pressure system over Florida that brought warm, moist air into Alabama.

Key meteorological conditions leading to the storm include: (1) strong upper level north-to-south winds slowly steering a surface-level cold front into Mobile, (2) warm, moist air from the Gulf that was pulled into Mobile ahead of the cold front, (3) vertical wind shear,33 and (4) a strong jet stream aloft.

Storm Damage

Heavy rain caused flooding in the streets of downtown Mobile, submerging vehicles,34 and overwhelmed two wastewater pumping stations, causing over 13 million gallons (49 million liters) of sewage to spill into Mobile Bay.35 The storm also downed trees and power lines, causing 7,600 homes to lose power.36 Across Alabama, resulting tornados damaged trees and buildings.

Extreme Event Comparison

Observed 24-Hour Precipitation: approximately a 5 to 15 year event

Observed Peak Discharge: less than a 2 year event

Observed Storm Surge: less than a 10 year event

Case Study 4: Hurricane Georges, September 28, 1998

Storm Track and Intensification

Hurricane Georges began as a tropical depression on September 15, 1998, four hundred miles south-southwest of Cape Verde.37 As the storm traveled westward, it steadily intensified, developing into a tropical storm on September 16, reaching hurricane strength by September 17, and peaking on September 19, as a Category 4 storm with winds of 150 miles per hour (240 kilometers per hour).38 Hurricane Georges caused damage in Puerto Rico, the Dominican Republic, Haiti, and Cuba, weakened at one point by the mountainous terrain of the Dominican Republic and Haiti.

Hurricane Georges entered the Gulf of Mexico on September 25, traveling north-northwest at an average speed of 11 miles per hour (18 kilometers per hour).39 The storm began to strengthen as it moved into the warm waters of the Florida Straits moving in a west-northwest track. Sea surface temperatures in the Gulf near the track of Hurricane Georges were estimated to be 81.7°F (27.6°C).40 This is close to the minimum sea surface temperatures of 82°F (28°C) typical for a storm to develop and maintain its strength.41

Georges made U.S. landfall near Biloxi, Mississippi around 6:30 am on September 28 as a Category 2 storm. The storm moved slowly over land and reached Mobile in the early morning of September 29.42 Because the storm moved so slowly, Alabama experienced significant torrential rains and coastal storm inundation.43

Storm Damage

Hurricane Georges caused severe flooding along the Gulf Coast from Mississippi to Florida, including the Mobile region. Downtown Mobile was heavily flooded as a result of heavy precipitation and high storm surge. This resulted in inundated and blocked roadways. The Mobile Bay Causeway was fully inundated, disabling
transportation across the bay between Mobile and Baldwin Counties.

Extreme Event Comparison

Observed 24-Hour Precipitation: approximately a 10 year event

Observed Peak Discharge: 25 year event

Observed Storm Surge: above a 10 year event

Case Study 5: Hurricane Katrina, August 29, 2005

Storm Track and Intensification

Hurricane Katrina was one of the most destructive hurricanes to hit the United States.44,45 The storm formed from the combination of a tropical wave, an upper-level trough, and the mid-level remnants of Tropical Depression Ten.46 Hurricane Katrina began its early development on August 23 as a tropical depression about 175 miles (280 kilometers) southeast of Nassau, Bahamas.47 On August 24, the tropical depression became a tropical storm as it moved towards the Bahamas. 48 In the early evening of August 25, the storm strengthened to a Category 1 hurricane with sustained winds of 80 miles per hour (128 kilometers per hour) before making landfall in Florida between Hallandale Beach and North Miami Beach. 49 Hurricane Katrina crossed the tip of Florida overnight and began to re-intensify over the warm Gulf waters (sea surface temperatures were 2°F to 4°F (1°C to 2°C) above normal).50

From August 25 to August 31, Hurricane Katrina slowly turned north-northwest. As Hurricane Katrina moved again towards landfall, Katrina intensified due to upper atmosphere conditions, above-normal sea surface temperatures, and less-than-normal vertical wind shear. On August 28, Hurricane Katrina became a Category 5 hurricane with peak winds speeds near 175 miles per hour (280 kilometers per hour) and a central pressure of 902 millibars. The storm extended about 105 miles (168 kilometers) from its center, with tropical storm force winds extending out another 100 miles (160 kilometers).

Figure 60: Storm Track and Infrared Image of Hurricane Katrina

On the morning of August 29, Hurricane Katrina made landfall in Plaquemines Parish, Louisiana as a strong Category 3 hurricane with wind speeds of about 127 miles per hour (203 kilometers per hour) and a central pressure of 920 millibars. After returning back to sea, Hurricane Katrina made its final landfall near the Louisiana-Mississippi border with winds reported at near 121 miles per hour (194 kilometers per hour).

Storm Damage

Extreme Event Comparison

Observed Precipitation: less than a 2 year event

Observed Peak Discharge: less than a 2 year event

Observed Storm Surge: 25 year event

Mobile County experienced significant damage from Hurricane Katrina, primarily in the form of coastal flooding and storm surge. Storm surge on Dauphin Island destroyed or damaged dozens of homes.51 In the city of Mobile, flood depths of 11 to 12.5 feet (3.4 to 3.8 meters) caused severe inundation and incapacitation of most major
roadways.52 Downtown Mobile was entirely inundated, causing authorities to issue a dusk-to-dawn curfew. The Mobile Bay Causeway was fully inundated, disabling transport across the bay.53 Katrina also caused debris damage from oil rigs in the Mobile area. Dauphin Island experienced damage from an offshore oil rig that washed up on the shore. An oil rig under construction along the Mobile River was dislodged and carried 1.5 miles (2.4 kilometers) north where it struck the Cochrane Bridge just north of downtown Mobile.54

7.2. Projected Storm Events

An analysis of future storm events was conducted to evaluate how storms could change in the future, and how Mobile's transportation could be exposed to storm surge. This section describes the methodology and key findings for the analysis of future storm events in the Mobile region. The analysis of future storm events is presented in two sections, corresponding to the two analyses that were conducted:

A literature review was conducted to help inform understanding of how storms could change in the Mobile region in the future due to climate change.

Methodology

The analysis of historical storm events experienced in the Mobile region highlighted which atmospheric phenomena contributed to the severity of each storm event. To help to characterize future storm events, a literature review of studies projecting how these atmospheric phenomena may change was conducted. This review provides clues as to how the frequency, duration, and intensity of storm events in the Mobile region may change.

Key Findings

Key Findings for Storm Event Literature Search

Many studies suggest a poleward shift of the jet stream, which would reduce the frequency of some mid-latitude storms around Mobile.

It is difficult to predict the impacts of climate change on hurricane activity due to conflicting changes in atmospheric phenomena.

This section presents key findings from the literature review of studies projecting how storm-related atmospheric phenomena affecting Mobile may change. The findings are presented in two parts: (1) severe thunderstorms and seasonal events, and (2) tropical storms and hurricanes. Appendix D.7 presents an overview of how storm events may change in the United States and globally.

Mid-latitude storms and thunderstorms

While studies project an overall increase in extra-tropical storm severity in the eastern United States,55 no studies were found that focused specifically on the Southeastern United States or the Mobile region. Therefore, future changes in Mobile storm events were investigated through studies that discuss how the atmospheric phenomena affecting the storm events may change.

Jet Stream. The jet stream can steer and intensify severe thunderstorms affecting Mobile. Many studies suggest a poleward shift of the jet stream. This would result in a northward shift in the mid-latitude storm track and reduce the frequency of some mid-latitude storm activity around Mobile.56

Convective Activity. Scientists suggest that convective storms may increase in intensity due to increased atmospheric moisture content, and frequency due to increasing summer minimum temperatures.57 Trapp et al. (2007)58 projects an increase in the environmental conditions conducive to severe thunderstorms in the spring and summer in the Mobile area. For example, vertically integrated buoyant energy and specific humidity are projected to increase with minimal increase in vertical wind shear. For the Mobile region, Figure 61 illustrates the increase in the number of days of severe thunderstorm environment (NDSEV) from the 1962-1989 time period to the 2079-2099 time period for the spring months (up to 1 additional day) and summer months (more than 2 additional days).59

These findings suggest that Mobile may experience less mid-latitude storm events as the jet stream moves north, but that this decrease in activity may be compensated by an increase in the intensity and/or frequency of extreme localized convective activity.

Figure 61: Change (1962-1989 to 2079-2099) in the Number of Days with Local Formation of Thunderstorms that Could Produce Significant Winds, Hail, and/or Tornadoes, for a Moderately-High (A2) Emission Scenario in Spring (d) and Summer (h)

Source: Trapp et al. 2007

Hurricanes and Tropical Storms

As discussed in FHWA (2010), there is some disagreement amongst scientists about how tropical storms and hurricanes may change in response to changes in climate. Further, it remains uncertain whether past changes in tropical storm activity were influenced by natural variability or human activity.60 The development of these storms has been linked to the presence of two important factors: low vertical wind shear but high SST.61

Vertical Wind Shear.62 Vertical wind shear in the tropical Atlantic Ocean is projected to increase, which would reduce the development of tropical storms and hurricanes that reach the Gulf.63

Relative Sea Surface Temperature (SST). Under a moderate (A1B) emission scenario, SSTs in the Gulf of Mexico are projected to significantly warm by the end of the century, which could lead to increased intensification of tropical storms and hurricanes entering the Gulf.64

However, these two competing factors make it difficult for hurricane experts to conclusively agree on how hurricane activity may change.

The recent scientific consensus on hurricane activity suggests hurricanes may globally decrease in frequency but increase in intensity. This consensus suggests that the globally averaged intensity of storms originating in the tropics will increase by 2 to 11% by the end of the century but the globally averaged frequency will decrease by 6 to 34%.65 This suggests a future decrease in overall hurricane number, but an increase in the severity of the hurricanes that do develop. Peduzzi et al. (2012) found that over the next 20 years, the mortality risk associated with the projected changes in tropical storms and hurricane activity increases due to the increase in both the intensity of the storm and demographic pressures, despite the reduction in the frequency of these storms and the potential progression in development and governance.

7.2.2. Scenario-and Model-Based Analysis of Hurricane Storm Surge

A scenario-based analysis of storm surge from hurricanes was also conducted; this analysis sought to answer two main questions:

What are the implications of a moderate hurricane striking the region under a scenario of increased sea level?

What are the implications of a strike by a larger hurricane than the region has experienced in recent history?

Methodology

To answer these questions, the storm surge inundation from 11 plausible storm scenarios was modeled. These 11 scenarios were developed using Hurricane Georges and Hurricane Katrina—two damaging storms that affected Mobile in recent history—as base storms, and then adjusting certain characteristics of the storm parameters to simulate what could happen under alternate conditions. This scenario approach was used to manage the uncertainty in quantitatively estimating how increases in atmospheric greenhouse gas concentrations are linked to future changes in hurricane characteristics.66

Environmental implications of the selected storm scenarios were assessed using state-of-the-art quantitative models. The scenario- and model- based analysis included the following steps:

Selection of storm surge scenarios

Advanced circulation modeling

Advanced circulation model testing

Wave modeling

Exposure mapping

For more detail, see Appendix D.8.

Selection of Storm Surge Scenarios

The first step of the scenario-based analysis was to select scenarios to represent a wide range of storms that could plausibly strike Mobile. For this analysis, records from historic storms were selected to use as the basis in developing these storm scenarios. There were two main questions that the scenario-based analysis attempted to address:

What are the implications of a moderate hurricane striking the region under a scenario of increased sea level? According to the Gulf Coast Phase 1 report, planners in the Gulf Coast region can expect a Category 1 or 2 hurricane approximately once every five years.67 A set of scenarios was developed to examine the extent of flooding from such storms when exacerbated by sea level rise.

What are the implications of a strike by a larger hurricane than the region has experienced in recent history? Although the odds of an intense hurricane strike are difficult to determine, those odds are likely increasing.68 A set of scenarios was developed to examine the implications of hurricanes that are larger in magnitude than recently experienced in the study area, but that will become more likely in the future. This was done by selecting a storm that occurred relatively recently, and intensifying it using different methods (described below) and including the effects of sea level rise.

In selecting the storms, historical storms were chosen that met the following criteria:

Local tide gage data are available throughout most of the course of the storm.

Post-storm high water mark data are available in the Mobile area.

The storm approached the coast relatively perpendicularly.

The strengths of the storms and their storm surges were appropriate to the two questions being addressed.

After reviewing records of all land-falling hurricanes in the Mobile area over the past few decades, the 1998 Hurricane Georges was selected to address Question #1, and the 2005 Hurricane Katrina was selected to address Question #2.

Using Hurricanes Georges and Katrina as base storms, 11 storm scenarios (see Table 23) were developed by adjusting certain characteristics of the storm parameters to simulate what could happen under alternate conditions. For the Georges simulations, all four sea level rise scenarios (0 meters (0 feet), 0.3 meters (1.0 foot), 0.75 meters (2.5 feet), and 2.0 meters (6.6 feet)) were examined. For the Katrina simulations, the modeling considered different adjustments, including shifting the path of Katrina so that it hit Mobile directly, intensifying the storm, and adding in 0.75 meters (2.5 feet) of sea level rise. Subsidence was not included in the storm surge analysis scenarios. Two of the 11 scenarios were hindcasts of Georges and Katrina. They were used to validate the model and to serve as a basis from which to build the other 9 scenarios.

Figure 62: Original Track of Hurricane KatrinaThe image shows the observed track of Katrina used in the "Natural" scenarios. Each dot represents the approximate location of NOAA's National Hurricane Center 6-hour advisory bulletin used in the model simulations. kph = knots per hour. Times are UTC.

Figure 63: Shifted Track of Hurricane KatrinaThis image shows the shifted track of Katrina that corresponds to the five "shift" scenarios explored in this study.

Advanced Circulation Modeling

Simulations of storm-induced water levels (i.e. storm surge) were performed using the ADvanced CIRCulation model, ADCIRC.73 This finite-element hydrodynamic code is robust, well-developed, extensively-tested, and highly adaptable to a number of coastal-ocean processes. The storm simulations were performed using the two-dimensional, depth integrated (2DDI) form of ADCIRC assuming barotropic forcing only (i.e. no density-driven flows). While the ADCIRC model is capable of applying a variety of internal and external forcings, including tidal forces and harmonics, inflow boundary conditions, density stratification, and wave radiation stresses, only the meteorological forcing input is used here to drive the storm-induced flows and water levels.

The ADCIRC storm simulations are driven by meteorological forcing data extracted from six-hour advisory forecast and observation reports issued by the NOAA National Hurricane Center (NHC). Meteorological data must be assembled in a modified Automated Tropical Cyclone Forecast (ATCF) best track format. An asymmetric hurricane vortex formulation74 based on a Holland-type gradient wind model75 is used to estimate the wind and pressure field of the storm. The Garratt (1977) formula is used to convert wind speed to an applied wind stress. These data are spatially interpolated onto the ADCIRC mesh (see Appendix D.8 for more information), and a linear interpolation is used to map six-hour advisory data to each intermediate time that the model performs its calculations76 falling between advisory information. A general schematic of this process is provided in Figure 64.

Hindcast simulations of storm-induced water levels using the ADCIRC hydrodynamic model were completed for Hurricanes Georges and Katrina to evaluate the model's ability to accurately reproduce the spatial distribution and peak storm-induced water levels of historical events. Results for ADCIRC are reported relative to Mean Sea Level. See Appendix D.8.3 for a description of testing.
Differences between the hindcast simulations and observations may be attributed to a number of simplifications, or assumptions, applied to the model scenarios or to deficiencies in the hydrodynamic model itself. These possible causes are listed below and described in detail in Appendix D.8.3:

The hindcast simulations do not include the effects of the tide.

The hindcast simulations do not include the effects of waves and wave breaking78.

The hindcast simulations do not consider watershed contributions to the simulated storm surge hydrograph.

The meteorological forcing used to drive the hindcast scenarios is a gross simplification of historical weather conditions and is limited further by the estimations of storm characteristics provided by the NHC advisory bulletins.

Wave Modeling

The wave characteristics accompanying each of the storm surge scenarios were simulated using a state-of-the-art model, STeady State spectral WAVE (STWAVE). It is a flexible, robust model for nearshore wind-wave growth and propagation. It is one of the most widely used models to compute waves in coastal environments, based on wind and bottom topography.

For each scenario, the STWAVE model was run following the ADCIRC model. The coupling between the models was asynchronous. In other words, the models were run separately and the wave fields did not influence surge estimates.

The wind fields used to drive STWAVE were derived from the Holland-type model that was used to drive the ADCIRC model. Waves were simulated over both open water and the land simulated to be inundated.

Dauphin Island currently helps to protect the mainland by attenuating waves generated out in the open Gulf. Some of that attenuation may be diminished if the topography of the island is reduced through erosion from prior storm wave action or through human actions. Following the 2010 Gulf oil spill, sediment was dug out from parts of the north side of Dauphin Island to build a berm on the south side, which was intended to keep oil from washing ashore. This had the effect of reducing the width of the island in places, which may have left it more vulnerable to breaching in future storms.79 These and potential future changes in morphology of the island are not taken into account in the simulations performed in this study.

Exposure Mapping

Finally, a Geographic Information System was used to overlay inundation under each of the storm surge scenarios on top of the critical assets defined in Task 1 of the Gulf Coast Study. This analysis accounts for the projected surge level and the elevations of each asset. This analysis considered the bare earth elevation of assets-that is, the elevation of the land on which the assets sit. It did not consider the height of the assets themselves.

Key Findings

Key Findings for Storm Surge Modeling

Projected exposure of critical transportation assets to storm surge is much greater than exposure to long-term sea level rise.

Future storm surge has the potential to greatly exceed any historical surges.

The magnitude of the highest sea level rise scenario examined in this study is lower than the range of flooding that may occur due to future hurricanes. However, sea level rise will invariably increase the area of flooding from coastal storms.

Critical port facilities are most exposed to storm surge, with the highest fractional extent of exposure.

Pipelines are least exposed, with the lowest fractional extent of exposure.

This section presents the key findings from the scenario-based analysis of hurricane storm surge and waves. Results are presented in a series of figures and a table at the end of this section.

Figure 65 through Figure 75 present maps of the storm surge results produced by the ADCIRC model under each of the scenarios indicated Table 23. The storm surge maps indicate the depth of inundation relative to current dry ground. They also show the infrastructure deemed to be critical in Task 1 of this project.80

Table 24 shows the maximum water elevation at the ADCIRC node closest to the NOAA tidal station at the Mobile Docks.

Figure 76 through Figure 86 show the wave modeling results that correspond to the storm surge simulations. The waves simulated here exacerbate the surge: they represent the significant wave heights above the still-water level of the corresponding surge. In other words, the wave heights may be added to the surge heights shown in Figures 65 through 75. We show the two separately, in part, to illustrate the difference in the wave heights and surge. The effect of the waves will be quantitatively assessed in a subsequent task that will account for the effect of their kinetic energy on transportation structures as well as their contribution to scour.

As noted earlier in the Advanced Circulation Model Testing section of this report, the "natural" simulations of Georges and Katrina indicate relatively similar surge depths and extents. The maximum flooding depth at the Mobile Docks gage was simulated to be 11.32 feet (3.43 meters)81 above mean higher high water (MHHW82) in Georges and 12.41 feet (3.76 meters) above MHHW in Katrina (see Table 24).

This degree of flooding generated by these "natural," unadjusted hurricanes is somewhat greater than the inundation from even the most extreme long-term sea level rise scenario (2.0 meters) considered in this report (see Figure 55). The flooded areas include all of the coastal wetlands in Mobile County, as well as Gaillard Island, Terrapin Island, and nearly all of Dauphin Island.83 Some of the low-lying areas along the waterfront and ports would also be inundated.

Wave heights are estimated at a few meters along the open bay shoreline and the open ocean, as well as in the wetlands to the north of I-10. Similar conditions are estimated for other wetlands with a direct fetch and close proximity to the ocean or bay. Wave heights in more inland inundated areas are estimated to be a meter or less. In general, wave heights will tend to scale in proportion to the depth of the water over the inundated land (lower depth implies lower wave heights).

A few interesting features are evident in all of the wave simulations. First, both Dauphin Island and Fort Morgan play a major role in reducing the wave energy entering Mobile Bay and striking the mainland. The reduction in wave heights from the south to the north sides of Dauphin Island and Fort Morgan is readily apparent. Second, the triangularly shaped low-wave feature to the southeast of Mobile Downtown Airport is created by the protective properties of Gaillard Island as well as the deeper water of the Bay's shipping channels that produces less wave shoaling. The main shipping channel can be seen bisecting the east part of the Bay's wave field from the west side.

The "Sea level Rise" Surge Simulations

The 0-meter (0 foot), 0.3-meter (1.0 foot), 0.75-meter (2.5 feet), and 2.0-meter (6.6 feet) GSLR scenarios for Georges were designed to address the question, what are the implications of a moderate hurricane striking the region under a scenario of increased sea level?

The analysis indicates that there are not large-scale difference between the "natural", 0.3-meter (1.0 foot), and 0.75-meter (2.5 feet) Georges simulations. There are, however, distinctions that are likely noteworthy for transportation. For example, sea level rise could expand the flooded area downtown.

In the 2.0-meter (6.6 feet) Georges simulation, nearly all of the central downtown area is under water. The number of evacuation routes that would be under water also increase significantly. Table 24 indicates that the inundation levels at Mobile Docks correspond quite closely to the amount of assumed GSLR. This finding indicates that rather than performing additional ADCIRC model runs with multiple sea level rise inputs, higher water levels could have simply been added on to the original Georges storm simulation to generate relatively similar maps. This study did not rigorously assess the geographic applicability of this conclusion. However, for the purposes of a first-order analysis, it is likely a robust conclusion.

The "Intense" Surge Simulations

All of the Katrina shifted path scenarios were designed to address the question, what are the implications of a hurricane striking the region that is larger than any in Mobile's historical record?

The maximum surge elevation at Mobile Docks from the "shifted" Katrina is 7.03 feet (2.13 meters) greater than the natural Katrina simulation. The magnitude of this surge corresponds very roughly to the magnitude of surge estimated from the Georges 2.0-meter (6.6 feet) scenario: 19.44 feet (5.89 meters) vs. 17.99 feet (5.45 meters). In addition, it is approximately what would be expected from the Katrina "natural" storm were it to occur on top of 2 meters of LSLR. In the shifted Katrina scenario, roughly a third of the area to the east of I-65, north of the downtown airport, and south of Chickasaw is inundated, as well as most of the area in Mobile County to the southeast of Bayou La Batre.

If the shifted Katrina scenario were to occur with sea level 0.75 meters (2.5 feet) higher, the surge at Mobile Docks is estimated to be 22.74 feet (6.89 meters). In addition to the flooding described above, nearly the entire stretch of Route 193 north of Mon Louis would be inundated. In addition, bands of flooding would reach west of downtown nearly to I-65.

If the shifted Katrina storm were to be more intense at landfall than the original storm, as per the "MaxWind" scenario (in which the maximum sustained wind speed at landfall is 150 knots), the surge at the Mobile Docks is estimated at 27.65 feet (8.38 meters). In this case, nearly all of the land to the east of I-65 would become flooded. Moreover, the water depths would be so great in many coastal areas that are currently dry ground that the waves could reach a few meters in height. Thus, structures more than 33 feet (10 meters) above sea level could be affected, including the downtown airport runways and hangars.

If the baseline local sea level under the "MaxWind" scenario was 0.75 meters (6.6 feet) higher, the surge at Mobile Docks is estimated at 31.02 feet (9.40 meters) and the inundation impacts would be correspondingly greater. Under the more conservative "ReducedPress" scenario (in which the central pressure84 of the shifted Katrina storm is reduced according to Knutson and Tuleya, 2004), the surge at Mobile Docks is estimated to be 24.85 feet (7.53 meters).

Note that increases in global sea level will not necessarily cause a corresponding one-to-one increase in peak storm surge elevations at all locations due to such factors as: non-linear variations in the forces increasing storm surge (such as wind setup) and forces resisting storm surge (such as bottom friction).

The depths shown are the height of the waves relative to the still-water level of the surge.

Caveats, Gaps, and Replicability

Not all factors affecting storm surge were taken into account in this study. For example, the study did not account for river flooding that often accompanies strong storms and tends to contribute to storm surge. Nor did it account for changes in beach profiles. For a more thorough account of the caveats, gaps, and replicability of this study, as well as lessons that may be useful in extending the results to other locales, see Appendix D.10.

7.3. Implications for Transportation

Storm surge can have very significant impacts on transportation, rendering them unusable for the duration of the surge (lasting several hours or more). Critical facilities – including roads, bridges, rail lines, airports and ports - may be unusable, or reduced in capacity, even after the waters recede due to damage to infrastructure, supporting utilities and communications, or access routes. Damage can range from debris that needs to be removed, to complete destruction of certain assets. The direct costs of clean up, repair and replacement can be high, and the secondary implications of disrupted transportation networks and supply chains can have widespread impacts on community life and on the local and regional economy.

The extent of inundation of critical transportation assets from storm surge is much greater than exposure to long-term sea level rise. Table 25 below was generated by using a Geographic Information System to overlay each of the storm surge scenarios over the critical assets defined in Task 1. The analysis takes into account the elevation on which each asset sits.

Based on fractional extent of exposure, critical port facilities are most exposed to storm surge. At least 92% of the 26 critical port facilities are inundated in all of the scenarios. In some of the most extreme scenarios, all of the critical port facilities are inundated.

In contrast to the port facilities, pipelines have the lowest fractional extent of exposure, ranging from 3% of pipeline-kilometers under the lowest scenario to 16% in the highest. Note that the pipeline data used in this analysis did not identify whether a particular section was above or below ground—a feature that would have a significant impact on the sensitivity of that section to inundation. Moreover, it also did not identify the exposure of pumping stations.

Most of the area's critical rail lines are close to the water, since a vast majority of them serve the port. According to this analysis, between 57% and 80% of the critical rail-kilometers would be exposed to storm surge under these scenarios.

Under the range of scenarios, exposure varies notably for critical roadways. In the lowest surge scenario, 27% of the critical roadway length is exposed, whereas in the most extreme scenario, 75% of the critical roadway length is exposed. Importantly, even in the lowest scenario, many of the key evacuation routes are affected. The large increase in exposure under the highest scenarios is due in part to the concentration of critical roadways between I-65 and downtown Mobile.

One of the two critical transit facilities, the GM & O Transportation Center, is located near the coast and inundated under all storm scenarios.

Of the two critical airports in the study area, only Mobile Downtown Airport is inundated under any of the storm surge scenarios. Under the lowest storm surge scenario, 4% of the airport's surface area is inundated, while the entire airport is inundated under the highest storm surge scenario. The scenarios in which the intensities of Georges and Katrina were not increased do not lead to major impacts on airport operations. However, the scenarios in which the track of Katrina is shifted would expose key aspects of the airport's operations to inundation.

Table 25: Critical Assets Inundated Under Each Storm Scenario

Scenario

Roads(mi)

Rail(mi)

Pipe-lines (mi)

Ports(#)

Transit Facilities (#)

Mobile Downtown Airport (mi2)*

Georges-Natural

55 of 209
(27%)

111 of 196
(57%)

14 of 426
(3%)

24 of 26
(92%)

1 of 2
(50%)

0 of 3
(4%)

Katrina-Natural

58 of 209
(28%)

116 of 196
(60%)

15 of 426
(3%)

24 of 26
(92%)

1 of 2
(50%)

0 of 3
(5%)

Georges-Natural-30cm

58 of 209
(28%)

114 of 196
(59%)

15 of 426
(3%)

24 of 26
(92%)

1 of 2
(50%)

0 of 3
(5%)

Georges-Natural-75cm

63 of 209
(30%)

119 of 196
(61%)

24 of 426
(6%)

24 of 26
(92%)

1 of 2
(50%)

0 of 3
(7%)

Georges-Natural-200cm

83 of 209
(40%)

132 of 196
(68%)

50 of 426
(12%)

24 of 26
(92%)

1 of 2
(50%)

0 of 3
(15%)

Katrina-Natural-75cm

69 of 209
(33%)

127 of 196
(65%)

44 of 426
(10%)

24 of 26
(92%)

1 of 2
(50%)

0 of 3
(9%)

Katrina-Shift

95 of 209
(46%)

140 of 196
(71%)

51 of 426
(12%)

24 of 26
(92%)

1 of 2
(50%)

2 of 3
(65%)

Katrina-Shift-75cm

114 of 209
(55%)

144 of 196
(73%)

54 of 426
(13%)

25 of 26
(96%)

1 of 2
(50%)

2 of 3
(90%)

Katrina-Shift-MaxWind

140 of 209
(67%)

150 of 196
(77%)

62 of 426
(15%)

26 of 26
(100%)

1 of 2
(50%)

3 of 3
(100%)

Katrina-Shift-MaxWind-75cm

149 of 209
(75%)

154 of 196
(79%)

67 of 426
(16%)

26 of 26
(100%)

1 of 2
(50%)

3 of 3
(100%)

Katrina-Shift-ReducedPress-75cm

124 of 209
(60%)

146 of 196
(74%)

56 of 426
(13%)

25 of 26
(96%)

1 of 2
(50%)

3 of 3
(98%)

Note: The "highly critical" asset list was revised after the criticality report was completed to include parts of CR188, CR59, and the Cochrane Bridge in response to comments received from local stakeholders. Therefore, the total km presented here may differ from that reported in the Criticality Assessment report.

The implications of the storm surge findings detailed in this report on transportation assets and services in Mobile will be investigated in the next task of this study (Task 3: Vulnerability Screen and Assessment).

18 Mobile, Alabama regularly experiences localized "air-mass" thunderstorms during the summer months. Though these storms can be problematic to the operations of the transportation system, the local weather service does not classify these thunderstorms as extreme.

24SURGEDAT divides the U.S. Gulf Coast into 10 regions. The data was constructed from 62 sources, including 28 Federal Government sources, numerous academic publications, and more than 3,000 pages of newspaper from 16 daily periodicals. For each region, the Southern Regional Climate Center (SRCC) linear regression method, a log-linear regression method, was utilized to estimate basin-wide and sub-regional surge water levels for the 10-year, 25-year, 50-year, and 100-year return periods. (Personal Communication with H.F. Needham, based on an analysis of data in Needham and Keim, 2011.)

25 Source: Personal Communication with H.F. Needham based on an analysis of data in Needham and Keim, 2011

42 Though Biloxi is just 60 miles from Mobile, they have different shoreline characteristics. Biloxi sits directly on the Gulf of Mexico, while Mobile is inset on Mobile Bay, with some barrier islands between the Gulf and the inlet. The differences may affect storm surge and so the locations are considered separately in this analysis.

58 The study provides projections of environmental conditions that support severe U.S. thunderstorms using a high resolution regional climate model under a moderately-high (A2) emission scenario.

59 These findings were compared to simulations of three climate models, MPI ECHAM5, GFDL CM2.1, and NCAR CCM3. All models demonstrated a similar directional trend for the Mobile region; however, the increase in NDSEV did vary from approximately 1 day in summer simulated by NCAR CCM3 to more than 3 days simulated by MPI ECHAM5. Overall, the findings provided in this study suggest an increase in NDSEV but with some uncertainty across models regarding the magnitude of the increase.

66 A scenario-based analysis is a standard approach in the face of "deep uncertainty" associated with environmental or other challenges relating to future conditions. The scenarios used in this analysis, which are reflective of the state-of-the-science, are not predictions. Rather, the scenarios represent conditions that may occur, thereby encompassing a representative range of possible future conditions.

69 The two questions being addressed are: (1) What are the implications of a moderate hurricane striking the region with a higher sea level? (2) What are the implications of a strike by a larger hurricane than the region has experienced in recent history?

70 The term "shift" indicates an eastward shift of the storm track. This is used to explore the potential for a direct hit of a major hurricane on the Mobile area. See Appendix D.8.1 for more details.

71 The term "ReducedPress indicates that the central pressure of the storm along its entire track was reduced by 14% according to the findings of Knutson and Tuleya (2004), which assessed the potential intensification of hurricanes due to an increase in atmospheric greenhouse gas concentrations. The central pressure of the storm is a measure of the storm's intensity: the lower the pressure, the more intense the storm. See Appendix D.8.1 for more details.

72 The term "MaxWind" indicates that the wind speeds were held constant at the values they had when the storm's maximum sustained wind speed of approximately 150 knots was recorded in the central Gulf of Mexico on August 28, 2005. See Appendix D.8.1 for more details.

80 The maps also indicate parts of CR188, CR59, and the Cochrane Bridge as critical, in response to comments received from local stakeholders.

81 Three significant digits are reported here for the sake of completeness in documentation. However, the variability across the scenarios and the uncertainty associated with the model is so great that for transportation planning purposes only a small amount of credence should be placed in the second digit. The third digit is generally only useful in illustrating differences between scenarios.

82 MHHW at the Mobile Docks gage is 1.2 feet above the NAVD88 vertical datum. Therefore, one must subtract about 1.2 ft from these elevations to obtain the corresponding elevations above NAVD88.

83 The western two-thirds of Dauphin Island is so thin that the ADCIRC mesh does not permit inundation of it in order to avoid numerical instabilities that might otherwise arise. Thus, although the maps of storm surge shown here do not explicitly indicate any flooding on the western two-thirds of the island, the reader should assume that it is flooded in all of the scenarios. For the same reason as western Dauphin Island, small islands along the coast have not been included.

84 The intensity of a hurricane is defined in part by its central pressure. The lower the central pressure, the more intense it generally is.

85 This figure and the following maps show the entire extent of the modeling domain, but do not show the entirety of Mobile County.

86 Critical port structures are not shown in this figure and the following maps since doing so at the scale of the modeling domain would make it difficult to read the map in the area of the ports. As discussed below, a large majority of the critical ports are inundated in all of the scenarios.

87 Figure 59 shows Georges' storm track approaching the Gulf Coast, where the color denotes the storm's Saffir-Simpson intensity rating (NOAA, 2011g). The image at the right is an enhanced infrared image of Georges that provides an illustrative demonstration of the shape and activity of the storm soon after hitting land (NOAA, 2011h).